Wide band dispersion-controlled fiber

ABSTRACT

A wide band dispersion-controlled fiber which comprises a core forming an optical signal transmission path and having a peak refractive index, and a cladding surrounding the core and having a peak refractive index lower than the peak refractive index of the core. The wide band dispersion-controlled fiber further comprises at least one dispersion control layer arranged between the core and the cladding and having a refractive index profile such that its refractive index increases from an inner periphery to an outer periphery. The minimum refractive index of the dispersion control layer is less than the peak refractive indices of the core and cladding.

PRIORITY

[0001] This application claims priority to an application entitled “WIDEBAND DISPERSION-CONTROLLED FIBER”, filed in the Korean IndustrialProperty Office on Nov. 30, 2001 and assigned Serial No. 2001-75152, thecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical fiber. Moreparticularly, the present invention relates to a dispersion-controlledfiber.

[0004] 2. Description of the Related Art

[0005] In general, the dispersion characteristics of an optical fibercan be effectively controlled by positioning a region of a depressedrefractive index between a core and a cladding of the fiber. This isdisclosed in U.S. Pat. No. 4,715,679 (title: “LOW DISPERSION, LOW-LOSSSINGLE-MODE OPTICAL WAVEGUIDE”) invented by and issued to Venkata A.Bhagavatula, the contents of which are incorporated by reference asbackground material.

[0006]FIG. 1 is a graph illustrating prior art dispersioncharacteristics of a single-mode fiber (SMF). In this illustration, adispersion curve 110 for the SMF is shown. The SMF has a step-indexprofile because there is no region having a depressed refractive index.As seen from the dispersion curve 110, the SMF has a unit dispersionvalue of about 17 ps/nm/km at a wavelength of 1550 nm. If the SMF isused for a long distance transmission, an accumulated dispersion of anoptical signal received through the SMF is increased and, as a result, adistortion of the optical signal becomes more severe. There are variousdispersion compensation techniques in the prior art for minimizing theaccumulated dispersion occurring during the long distance transmissionof the optical signal. Generally, a method of using adispersion-controlled fiber has been widely employed to minimize theaccumulated dispersion.

[0007] Dispersion-controlled fiber has a high negative dispersion valuebecause of a depressed refractive index region surrounding its core.Further, the dispersion-controlled fiber can be connected to one end ofthe SMF to compensate for the accumulated dispersion of the SMF. Thedispersion-controlled fiber has a high negative unit dispersion value ata wavelength of 1550 nm and its length may be adjusted to offset theaccumulated dispersion of the SMF, so that the total dispersion becomeszero.

[0008] However, if the dispersion-controlled fiber is adapted fordispersion compensation of the SMF, a sum of an accumulated dispersionof the dispersion-controlled fiber and the accumulated dispersion of theSMF may not be zero at wavelengths other than 1550 nm. In this regard,there is a problem in which it is not appropriate to apply thedispersion-controlled fiber to a wavelength division multiplexingsystem.

[0009] In order to overcome the above problem, research has recentlybeen done to provide a fiber capable of compensating for both adispersion and a dispersion slope together. To compensate for both thedispersion and dispersion slope, it is required to let a dispersionvalue and dispersion slope of the SMF be D_(SMF) and DS_(SMF) and thoseof the dispersion-controlled fiber be D_(DCF) and DS_(DCF),respectively, such that the D_(DCF) and DS_(DCF) satisfy the followingequation 1.

D_(SMF):DS_(SMF)≅D_(DCF):DS_(DCF)  [Equation 1]

[0010] If the dispersion and dispersion slope (D_(DCF) and DS_(DCF) ) ofthe dispersion-controlled fiber satisfy equation 1, compensation for theaccumulated dispersion of the SMF occurs not only at a wavelength of1550 nm, but also at wavelengths other than 1550 nm. However, there is agreat deal of difficulty implementing a fiber that perfectly satisfiesequation 1 over the entire wavelength range. For this reason, thecurrent state of the art simply compensates for the dispersion anddispersion slope at C-band wavelengths of 1530-1570 nm. In a wide bandwavelength division multiplexing system, there is a need to perform thedispersion and dispersion slope compensations at any wavelength in arange of wavelengths including an S-band of 1450-1530 nm and L-band of1570-1610 nm as well as the C-band.

SUMMARY OF THE INVENTION

[0011] Therefore, the present invention provides a dispersion-controlledfiber applicable to a wide band wavelength division multiplexing system,with such a wide band wavelength being heretofore unknown in the art.

[0012] In accordance with the present invention, the above and otherobjects can be accomplished by providing a wide banddispersion-controlled fiber comprising a core forming an optical signaltransmission path and having a peak refractive index, and a cladsurrounding the core and having a peak refractive index lower than thepeak refractive index of the core, further comprising at least onedispersion control layer arranged between the core and the cladding andhaving a refractive index profile such that its refractive index isincreased from an inner periphery of the dispersion control layer havinga minimum refractive index lower than the peak refractive indices of thecore and cladding to its outer periphery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0014]FIG. 1 is a graph illustrating conventional dispersioncharacteristics of a single-mode fiber;

[0015]FIG. 2 is a view showing a structure and refractive index profileof a wide band dispersion-controlled fiber in accordance with a firstembodiment of the present invention;

[0016]FIG. 3 is a view showing a structure and refractive index profileof a wide band dispersion-controlled fiber in accordance with a secondembodiment of the present invention;

[0017]FIG. 4 is a view showing a structure and refractive index profileof a wide band dispersion-controlled fiber in accordance with a thirdembodiment of the present invention;

[0018]FIG. 5 is a view illustrating a function of the wide banddispersion-controlled fiber in FIG. 2;

[0019]FIG. 6 is a graph illustrating dispersion characteristics of thewide band dispersion-controlled fiber in FIG. 2;

[0020]FIG. 7 is a graph illustrating an example of compensating for adispersion of a single-mode fiber using the wide banddispersion-controlled fiber in FIG. 2; and

[0021]FIG. 8 is a view illustrating a process of manufacturing a preformof the wide band dispersion-controlled fiber in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Now, preferred embodiments of the present invention will bedescribed in detail with reference to the annexed drawings. In thefollowing description, a variety of specific elements such asconstituent elements are described. The description of such elements hasbeen made only for a better understanding of the present invention.Those skilled in the art will appreciate that various modifications,additions, and substitutions to the specific elements are possible,without departing from the scope and spirit of the invention asdisclosed in the accompanying claims.

[0023]FIG. 2 illustrates a structure and a respective refractive indexprofile of a wide band dispersion-controlled fiber in accordance with afirst embodiment of the present invention. As shown in this drawing, thewide band dispersion-controlled fiber 200 has a core 210, adispersion-controlled layer 220 and cladding 230.

[0024] The core 210 is arranged in the center of the wide banddispersion-controlled fiber 200 and has a radius of A₁ and a refractiveindex of N₁. The core 210 is bar-shaped and has a dispersion profile isset to a constant value N₁. A general formula for the refractive indexprofile is expressed as in the following equation 2. $\begin{matrix}{{N(R)} = {N_{1}\left\lbrack {1 - {2{\Delta_{1}\left( \frac{R}{A} \right)}^{\alpha_{1}}}} \right\rbrack}^{1/2}} & \left\lbrack {{Equation}\quad 2} \right\rbrack\end{matrix}$

[0025] where, R(≦A) is a diametrical distance, A(≦A₁) a diametricaldistance to a certain point within the core 210, N(R) a refractive indexaccording to the R, N₁ a peak refractive index of the core 210, Δ₁ afirst refractive index difference and α₁(0<α₁≦∞) a first shape indexdetermining a shape of the refractive index profile. Further, the firstrefractive index difference can be expressed as in the followingequation 3. $\begin{matrix}{\Delta_{1} = {\frac{\left. {N_{1}^{2} - N_{2}^{2}} \right)}{2N_{1}^{2}} \approx \frac{\left( {N_{1} - N_{2}} \right)}{N_{1}}}} & \left\lbrack {{Equation}\quad 3} \right\rbrack\end{matrix}$

[0026] where, N₂ is a peak refractive index of the cladding 230.

[0027] If necessary, the N₂ in the equation 3 can be substituted for anyvalue less than the peak refractive index N₁ of the core 210 and morethan a minimum refractive index N₄ of the dispersion-controlled layer220.

[0028] The dispersion-controlled layer 220 is arranged between the core210 and cladding 230 and has an inner radius A₁, an outer radius A₃,peak refractive index N₃ and the minimum refractive index N₄. Thedispersion-controlled layer 220 further is tubeshaped and has arefractive index that increases linearly from its inner periphery to itsouter periphery. A refractive index profile of the dispersion-controlledlayer 220 can be expressed as the following equation 4. $\begin{matrix}{{N(R)} = {N_{4}\left\lbrack {1 - {2{\Delta_{2}\left( \frac{R}{A} \right)}^{\alpha_{2}}}} \right\rbrack}^{1/2}} & \left\lbrack {{Equation}\quad 4} \right\rbrack\end{matrix}$

[0029] where, the A(A₁≦A≦A₂) is a diametrical distance to any point inthe dispersion-controlled layer 220, R(A₁≦R≦A) a diametrical distance,N₄ the minimum refractive index of the dispersion-controlled layer 220,Δ₂ a second refractive index difference, α₂(0<α₂≦∞) a second shape indexdetermining a shape of the refractive index profile. Further, the secondrefractive index difference can be expressed by the following equation5. $\begin{matrix}{\Delta_{2} = {\frac{\left( {N_{4}^{2} - N_{3}^{2}} \right)}{2N_{4}^{2}} \approx \frac{\left( {N_{4} - N_{3}} \right)}{N_{4}}}} & \left\lbrack {{Equation}\quad 5} \right\rbrack\end{matrix}$

[0030] where, N₃ is a peak refractive index of the dispersion-controlledlayer 220.

[0031] The cladding 230 is arranged outside of the wide banddispersion-controlled fiber 200 and has a radius of A₃ and refractiveindex of N₂.

[0032] If necessary, the dispersion-controlled layer, according to thepresent invention, can be implemented in various shapes. This variety ofthe implemented shapes will be described below with second and thirdembodiments of the present invention.

[0033]FIG. 3 illustrates a structure and a respective refractive indexprofile of a wide band dispersion-controlled fiber in accordance withthe second embodiment of the present invention. As shown in thisdrawing, the wide band dispersion-controlled fiber 300 has a core 310,dispersion-controlled layer 320 and cladding 330.

[0034] The core 310 is arranged in the center of the wide banddispersion-controlled fiber 300 and has a radius of A₁ and a refractiveindex of N₁. The core 310 is bar-shaped and has a dispersion profilethat is set to a constant value N₁.

[0035] The dispersion-controlled layer 320 is arranged between the core310 and cladding 330 and has an inner radius A₁, outer radius A₃, peakrefractive index N₃ and minimum refractive index N₄. Thedispersion-controlled layer 320 further has a tube shape and itsrefractive index increases curvilinearly from the inner radius to theouter radius.

[0036] The cladding 330 is arranged outside of the wide banddispersion-controlled fiber 300 and has a radius of A₃ and refractiveindex of N₂.

[0037]FIG. 4 illustrates a structure and a respective refractive indexprofile of a wide band dispersion-controlled fiber in accordance withthe third embodiment of the present invention. As shown in this drawing,the wide band dispersion-controlled fiber 400 has a core 410,dispersion-controlled layer 420 and cladding 330.

[0038] The core 410 is arranged in the center of the wide banddispersion-controlled fiber 400 and has a radius of A₁ and a refractiveindex of N₁. The core 410 further is bar-shaped and its dispersionprofile is set to a constant value N₁.

[0039] The dispersion-controlled layer 420 is arranged between the core410 and cladding 430 and has an inner radius A₁, an outer radius A₃, apeak refractive index N₃ and a minimum refractive index N₄. Thedispersion-controlled layer 420 further has a tube shape and itsrefractive index increases step-wise from its inner periphery to itsouter periphery.

[0040] The cladding 430 is arranged outside of the wide banddispersion-controlled fiber 400 and has a radius of A₃ and a refractiveindex of N₂.

[0041]FIG. 5 illustrates a function of the wide banddispersion-controlled fiber 200 shown in FIG. 2. This drawing showsintensity curves 510 and 520 for optical signals of shorter and longerwavelengths, which travel through the dispersion-controlled fiber 200.Namely, the curves 510 and 520 represent optical signal intensityprofiles corresponding to a certain cross section of the wide banddispersion-controlled fiber 200.

[0042] As seen from the intensity curve 510 for the shorter wavelengthoptical signal, a peak intensity point of the curve 510 is almostidentical to the center of the core 210 and the intensity profile isconcentrated at a core position. In other words, where the shorterwavelength optical signal travels through the wide banddispersion-controlled fiber 200, the amount of this optical signal whichpenetrates into the dispersion-controlled layer 220 is relatively smalland most of the optical signal travels in the core 210. As a result, thedispersion-controlled layer 220 has a relatively small effect on theshorter wavelength optical signal, in connection with dispersion.

[0043] As seen from the intensity curve 520 for the longer wavelengthoptical signal, a peak intensity point of the curve 510 is almostidentical to the center of the core 210 and the intensity profile isdispersed over positions of the core 210 and dispersion-controlled layer220. In other words, the longer wavelength optical signal penetratesinto the dispersion-controlled layer 220 in a relatively great amount asit travels through the wide band dispersion-controlled fiber 200 and aconsiderable part of the optical signal travels through thedispersion-controlled layer 220. As a result, the dispersion-controlledlayer 220 has a relatively great effect on the longer wavelength opticalsignal, in connection with dispersion.

[0044] As a dispersion-characteristic control for the longer wavelengthoptical signal is made possible, it is possible to control thedispersion curves, according to wavelengths, for the wide banddispersion-controlled fiber 200. This control process will be describedstep by step below.

[0045] Firstly, a dispersion curve by wavelengths of a longer wavelengthband is set through controlling respective refractive index profiles ofthe core 210 and dispersion control layer 220 under the condition that arefractive index profile of the cladding 230 is set to a constant value.

[0046] Secondly, a dispersion curve by wavelengths of a shorterwavelength band is set through controlling a slope of a refractive indexprofile of the dispersion control layer 220.

[0047]FIG. 6 is a graph illustrating dispersion characteristics of thewide band dispersion-controlled fiber in FIG. 2. This drawing shows afirst dispersion curve 610 when the difference between the peakrefractive index N₃ and the minimum refractive index N₄ is zero, asecond dispersion curve 620 when the difference is 0.0005, a thirddispersion curve 630 when the difference is 0.001 and a fourthdispersion curve 640 when the difference is 0.0015.

[0048] The first to fourth dispersion curves 610,620,630 and 640 are sosimilar to each other that it is difficult to distinguish any one ofthem from the others in a shorter wavelength band. On the other hand,there is an apparent difference between those dispersion curves in alonger wavelength band, or at wavelengths of 1500 nm or more.

[0049] Referring to FIG. 7, a description will be given regarding amethod for compensating for a dispersion and a dispersion slope of asingle-mode fiber by controlling respective refractive indexes of thecore 210 and dispersion control layer 220 of the wide banddispersion-controlled fiber 200 shown in FIG. 2. FIG. 7 shows adispersion curve 710 of the single-mode fiber, a dispersion curve 720 ofthe wide band dispersion-controlled fiber 200 whose dispersion controllayer 220 is controlled to adjust its dispersion slope, and a dispersioncurve 730 representative of the total dispersion when the single-modefiber and wide band dispersion-controlled fiber 200 are interconnectedat a length ratio of 1:1. As seen from the total dispersion curve 730,the dispersion compensation can be accomplished for a wavelength regionincluding an S-band and L-band as well as a C-band using the wide banddispersion-controlled fiber 200.

[0050] As shown in FIGS. 6 and 7, by adjusting the dispersion slope ofthe dispersion control layer 220, the dispersion and dispersion slope ofthe dispersion-controlled fiber 200 are adjusted such that thedispersion-controlled fiber 200 has a negative dispersion value, therebybeing capable of compensating for the dispersion of the single-modefiber with the negative dispersion value over a wide band including theS-band, C-band and L-band.

[0051] With reference to FIG. 8, a description will be given regarding amethod for manufacturing a pre-form of the wide banddispersion-controlled fiber in FIG. 2. The fiber pre-form manufacturingmethod may be MCVD (Modified Chemical Vapor Deposition), VAD (VaporPhase Axial Deposition), OVD (Outside Vapor Phase Deposition), or soforth. Here, a method for manufacturing the fiber pre-form using theMCVD is described. Because the MCVD is a known art, only condensing andcollapsing processes are described.

[0052] A pre-form manufacturing apparatus comprises a raw material gassupplier 820, a shelf 850 and an oxygen/hydrogen burner 860.

[0053] The raw material gas supplier 820 acts to mix oxygen and aplurality of additives and supplies oxygen and raw material gas, such asSiCl₄, GeCl₄, POCl₃, CF₄, SiF₄ and so forth, to an inner part of a tube810. The GeCl₄and POCl₃ are used for raising a refractive index of adeposition region and the CF₄, and SiF₄ for reducing the refractiveindex of the deposition region. The raw material gas supplier 820appropriately adjusts amounts of oxygen and raw material gas flowing tothe tube 810 to obtain the refractive index profile as shown in FIG. 2.For example, in the case where the dispersion control layer 220 isdeposited, as the deposition process is repeatedly performed, the rawmaterial gas supplier 820 adjusts the ratio of CF₄or SiF₄, supplied tothe deposition tube 810, to the mixture of oxygen, SiCl₄, GeCl₄, andPOCl₃ to generate a desired slope of the refractive index. In the casewhere the core 210 is deposited, as the deposition process is repeatedlyperformed, the raw material gas supplier 820 adjusts the ratio of GeCl₄,supplied to the deposition tube 810, to the mixture of oxygen and SiCl₄to generate a change in the refractive index.

[0054] The shelf 850 has a pair of chucks 832 and 836 and a guide 840.The deposition tube 840 is rotatably fixed between the pair of chucks832 and 836. The guide 840 is movably mounted onto the oxygen/hydrogenburner 860.

[0055] The oxygen/hydrogen burner 860 is supplied with oxygen andhydrogen to apply heat to a periphery of the deposition tube 840 whilemoving along the guide 840 at a constant rate. As a result, a hightemperature region is formed at the inner part of the deposition tube840 and the formed raw material gas passes through the high temperatureregion to generate a reactant. An associated reaction formula may beexpressed by, for example, SiCl₄+O₂→SiO₂+2Cl₂ and GeCl₄+O₂→GeO₂+2Cl₂. Bymeans of a thermophoretic mechanism, the reactant moves to an inner wallof the deposition tube 810, which is at a relatively low temperature,and is then deposited on the inner wall of the deposition tube 810.

[0056] Although one dispersion control layer is provided in thedispersion-controlled fiber in the preferred embodiments of the presentinvention, multiple dispersion control layers can be arranged betweenthe core and the cladding of the dispersion-controlled fiber ifnecessary. An intensity profile dispersion of an optical fiber varieswith a wavelength from a shorter wavelength to a longer wavelength. Inthis regard, the multiple dispersion control layers can be employed whenthere is a need for a finer control of dispersioncharacteristic-by-wavelength of the wide band dispersion-controlledfiber.

[0057] As apparent from the above description, it is possible to controldispersion characteristics of the wide band dispersion-controlled fiberaccording to the present invention for a longer wavelength band usingthe refractive index profile of the dispersion control layer thereof. Asa result, the wide band dispersion-controlled fiber according to thepresent invention has an advantage in that it is applicable to a wideband wavelength division multiplexing system.

[0058] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A wide band dispersion-controlled fibercomprising: a core forming an optical signal transmission path andhaving a peak refractive index, a cladding surrounding the core andhaving a peak refractive index that is less than the peak refractiveindex of the core, at least one dispersion control layer arrangedbetween the core and the cladding, said at least one dispersion controllayer having a refractive index profile such that its refractive indexincreases from an inner periphery to an outer periphery, and whereinsaid at least one dispersion control layer has a minimum refractiveindex that is less than the peak refractive indices of the core andcladding.
 2. The wide band dispersion-controlled fiber as set forth inclaim 1, wherein the dispersion control layer has a peak refractiveindex less than the peak refractive index of the cladding.
 3. The wideband dispersion-controlled fiber as set forth in claim 1, wherein thewide band dispersion-controlled fiber has a negative dispersion value ina wavelength band of 1400-1650 nm.
 4. The wide-band dispersioncontrolled fiber as set forth in claim 1, wherein the where the core hasa radius A₁, a refractive index of N₁ and a dispersion profile is set toa constant value N₁, and refractive index profile of the core isexpressed according to the following equation:${N(R)} = {N_{1}\left\lbrack {1 - {2{\Delta_{1}\left( \frac{R}{\Lambda} \right)}^{\alpha_{1}}}} \right\rbrack}^{1/2}$

where, R(≦A) is a diametrical distance, A(≦A₁) a diametrical distance toa predetermined point within the core, N(R) a refractive index accordingto the R, N₁ a peak refractive index of the core, Δ₁ a first refractiveindex difference and α₁(0<α₁≦∞) a first shape index determining a shapeof the refractive index profile.
 5. The wide-band dispersion controlledfiber as set forth in claim 4, wherein the first refractive index isexpressed according to$\Delta_{1} = {\frac{\left( {N_{1}^{2} - N_{2}^{2}} \right)}{2N_{1}^{2}} \approx \frac{\left( {N_{1} - N_{2}} \right)}{N_{1}}}$

where, N₂ is a peak refractive index of the cladding.
 6. The wide-banddispersion controlled fiber according to claim 1, wherein said at leastone dispersion-controlled layer is tube-shaped, and wherein therefractive index of said at least one dispersion-controlled layerincreases linearly from an inner periphery to an outer periphery.
 7. Thewide-band dispersion controlled fiber according to claim 1, wherein therefractive index profile of said at least one dispersion-controlledlayer 220 can be expressed according to the following equation:${N(R)} = {N_{4}\left\lbrack {1 - {2{\Delta_{2}\left( \frac{R}{A} \right)}^{\alpha_{2}}}} \right\rbrack}^{1/2}$

where, the A(A₁≦A≦A₂) is a diametrical distance to any point in thedispersion-controlled layer 220, R(A₁≦R≦A) a diametrical distance, N₄the minimum refractive index of the dispersion-controlled layer 220, Δ₂a second refractive index difference, α₂(0≦α₂≦∞) a second shape indexdetermining a shape of the refractive index profile.
 8. The wide-banddispersion controlled fiber according to claim 7, wherein the secondrefractive index difference is expressed by the following:$\Delta_{2} = {\frac{\left( {N_{4}^{2} -_{3}^{2}} \right)}{2N_{4}^{2}} \approx \frac{\left( {N_{4} - N_{3}} \right)}{N_{4}}}$

where, N₃ is a peak refractive index of the dispersion-controlled layer.